1. Field of the Invention
The present invention relates to an image readout apparatus, and more particularly to an image readout apparatus having an improved, novel shading distortion elimination unit.
2. Description of the Prior Art
An image readout apparatus is commonly used in a presently prevailing facsimile apparatus.
Most of the conventional image readout apparatus have in general a circuit construction as shown in FIG. 1. In the figure, reference numeral 1 represents an image sensor such as a CCD sensor, to which light reflected by an original is applied. The fundamental circuit operation is: reflected light from a predetermined area (one line) of the original is sequentially applied to the image sensor 1 while either the original or the sensor is moved relative to each other; an output from the image sensor 1 is amplified by an amplifier 3 to compare it with a slice level at a variable resistor 15 by a comparator 17; and the comparison result is output as a binary signal.
As is well known in the art, however, since a shading distortion due to scattering of the light quantity of a light source or a readout error due to scattering of a sensor sensitivity may occur, a circuit section shown in the middle portion of the circuit of FIG. 1 is provided.
The output of the amplifier 3 is input to an A/D converter 9 and a peak hold circuit 13, the A/D converter 9 converting an output of the amplifier 3 into a digital value of a predetermined number of bits for outputting it to a memory 7. Data in the memory 7 can be restored into analog data through a D/A converter 11. The peak hold circuit 13 holds the maximum value of the read-out signal to supply its output 13a to the A/D converter 9 and D/A converter 11 as their reference values. The analog voltage restored by the D/A converter 11 is applied to variable resistor 15 and hence to the resistor 15 whereat a slice level at the comparator 17 for binarization appears.
Data transfer between the memory 7, A/D converter 9, D/A converter 11 and so on is under control of a readout control unit 5 constructed of a microcomputer and the like.
To compensate for shading distortion and scattering of sensitivity of each element of the image sensor 1 of the image readout apparatus constructed as above, a white reference surface of such as a white reference plate mounted on a preset position of the apparatus is scanned. The output of the image sensor 1 is read and its digital value obtained at the A/D converter 9 is temporarily stored in the memory 7. That is, the memory 7 stores a shading distortion signal. Then, in case of reading an original, data stored in the memory 7 is restored into an analog voltage at the D/A converter 11, which is divided at the variable resistor 15 to use the divided voltage as a slice level for binarization.
In reading an original, an output 13a of the peak hold circuit 13 which corresponds to the maximum brightness at one line of the original is used as a reference voltage of the D/A converter.
FIGS. 2A to 2D show an output waveform obtained by reading a white reference surface, and other voltage waveforms obtained while scanning an original.
FIG. 2A shows an output waveform obtained by reading a white reference surface. The digital values of the output waveform are stored in the memory 7. FIG. 2B shows a reflectance of an actual one-line original. The output 3a of the amplifier 3 corresponding to the reflectance is shown in FIG. 2C by a solid line. The peak hold circuit 13 holds a one-line peak value as shown by a broken line 13a. Based on the peak value 13a, the D/A converter reads the value stored in the memory 7 to obtain a value 11a. In this case, the peak hold value 13a and the output value of the sensor read at its maximum sensitivity position (center position) are not coincident, so that the peak hold value 13a is small. Also, the reference voltage 11a as well as a voltage 15b divided by the variable resistor 15 is small, to accordingly obtain an output of the comparator 17 as shown in FIG. 2D. A slice level for binarization should essentially be at 60% of an original reflectance. But in the above case, the slice level is set at 30 to 40% of the reflectance. According to the prior art, there arises a problem that a correct slice level, which is dependent on the scattering of shading distortion and sensor sensitivity, is not achieved unless the reflectance peak value of an original is obtained along one line at the position where the sensor output is maximum.
For instance, if the maximum output while scanning an original is obtained at the position where shading distortion and sensitivity is maximum (in this case, at the one-line center position of an original), then a correct slice level is obtained. However, as shown in FIG. 2B, if the maximum brightness of an original is at the end of one line of an original and is not obtained at the position where the sensor output is maximum, then the slice level becomes small and a normal binarization cannot be realized.